The invention relates to a liquid-crystal display device provided with a display cell comprising a layer of a liquid-crystal material sandwiched between a first polarizer having a first direction of polarization and a second polarizer having a second direction of polarization.
The invention further relates to a reflective polarizer having a polarization direction at a first surface.
The display device is used, for example, in instruments or (portable) communication equipment, such as mobile telephones.
A display device of the above-mentioned type is described in EP-A-0.606.939 (PHN 14.346). In said document, a transmissive display device is shown. The known display device is embodied so that a maximum amount of radiation (light) having the direction of polarization of the second polarizer is passed by a combination of a cholesteric filter and a xc2xcxcex-plate. To achieve this, the direction of polarization of the second polarizer is parallel to the direction of polarization of the radiation passed by the combination of the cholesteric filter and the xc2xcxcex-plate. The display device shown can only be used in the transmission mode.
It is an object of the invention to provide, inter alia, a display device which can be used in the transmission mode as well as in the reflection mode. The possibility of employing a display device in the reflection mode is desirable, particularly, in the case of display devices used in portable equipment, because it enables ambient light to be used, so that the supply source of the radiation source (generally an LED) has a longer service life. The invention further aims at providing such a transflective display device in which radiation from the radiation source (often constructed as a backlight or a sidelight) is optimally used.
To achieve this, a display device in accordance with the invention is characterized in that the display cell is provided with a reflective polarizer of which the direction of polarization on the side of the second polarizer makes an angle relative to the second direction of polarization and which has a polarization-changing effect on the side facing away from the second polarizer for light incident at an angle relative to the normal to the reflective polarizer.
In this application, the expression xe2x80x9creflective polarizerxe2x80x9d is to be taken to mean a polarizer which, on at least one side of the reflective polarizer, passes linearly polarized radiation (light) in the so-called direction of polarization of the reflective polarizer, and which reflects linearly polarized radiation (light) in a direction at right angles thereto. In the known device, from the other side, circularly polarized incident radiation (light) is passed in the one direction, and circularly polarized incident radiation (light) is reflected in the other direction. It is alternatively possible, however, that the reflective polarizer passes or reflects linearly polarized radiation (light) on both sides.
The expression xe2x80x9cpolarization-changing effectxe2x80x9d is to be understood to mean that the polarization state of radiation passing through the reflective polarizer changes (for example from circularly polarized to elliptically polarized or from elliptically polarized to linearly polarized, and even from linearly polarized back to circularly polarized in the opposite direction) and hence said radiation is passed at least partly, so that, after passing through the reflective polarizer, it is polarized, for example, in the direction of polarization of the second polarizer. This distinguishes this property from a so-called xe2x80x9cdepolarizing effectxe2x80x9d, which causes a polarization state to change in an uncontrolled manner or be lost completely, for example, as a result of irregularities at a surface or (local) variations, for example, in the refractive index (particularly at the surface). A good xe2x80x9cpolarization-changing effectxe2x80x9d is already achieved, for example, if circularly polarized radiation (light) incident at an angle relative to the normal to the reflective polarizer, for example, of 30 degrees, is converted to elliptically polarized radiation (light) having an elipticity of at least 2, the term elipticity being defined as the ratio between the length of the long axis and the short axis of the ellipse described by the electric field vector of the radiation.
As, in the known device, front-side radiation passed by the display cell is polarized in the direction of polarization of the second polarizer, this radiation is passed by the cholesteric filter. As a result, such a device is unsuitable for use in the reflection mode. In the device in accordance with the invention, the reflective polarizer reflects more or less radiation as a function of the angle between the directions of polarization of the reflective polarizer and the second polarizer. If the directions of polarization cross each other at right angles, complete reflection takes place. As a result, the display device can suitably be used (in reflection) in ambient light conditions.
The part originating from the radiation source, which is passed by the reflective polarizer as a result of the polarization-changing effect, is (partly) passed, in succession, by the second polarizer and by the display cell. This polarization-changing effect, which is avoided as much as possible in the known device because it leads to a reduction in contrast (not to complete extinction upon driving between crossed polarizers) is used in the device in accordance with the invention and, as will become apparent, even enhanced to obtain a second (transmissive) setting of the display device.
In the reflective polarizer, the change in polarization is greater as the radiation is incident at a larger angle relative to the normal. Therefore, a further display device in accordance with the invention is characterized in that the display device comprises a radiation source which mainly spreads radiation which is incident on the side of the reflective polarizer facing away from the liquid crystal at an angle relative to the normal to the reflective polarizer. For this purpose, use is made, for example, of a sidelight radiation source.
The reflective polarizer preferably comprises a cholesteric filter and a birefringent layer.
Cholesteric filters comprise an optical layer of a (polymerized) liquid-crystal material having a cholesteric order. This means that the molecules are ordered in accordance with a helical structure with a pitch p. If unpolarized radiation is incident on such a layer, a circularly polarized radiation component having a(n) (anti-clockwise or clockwise) direction of rotation corresponding to the direction of the helix, and a wavelength corresponding to the pitch p of the helix will be reflected, whereas a circularly polarized radiation component having the opposite direction of rotation and a wavelength which is not adapted to the pitch is passed.
The cholesteric filter may have either a broad band, because the cholesterically ordered liquid-crystal material has a variable pitch, or it may have a narrow band. The latter is favorable when use is made of a radiation source having a narrow wavelength range, such as an LED.
In a special embodiment, the birefringent layer is constructed as a xc2xcxcex-plate or a xc2xexcex-plate, the wavelength being adapted to that of the radiation source (in the case of a broad-band cholesteric filter, for example, the average wavelength of the wavelength range used is selected). By rendering the birefringent layer switchable (that is, the polarization-changing effect can be adjusted over at least a part of the thickness of the layer) a more efficient use of the radiation source in the transmissive state can be achieved. Depending on the ambient light, for example, the elipticity of the light passed and hence the contrast can be adapted. It is also possible to use the adjustability of the birefringent layer to eliminate variations in elipticity or undesirable residual elipticity caused by process variations.
A further embodiment of a display device in accordance with the invention is characterized in that, on the side of the second polarizer, the reflective polarizer is provided with a polarization-preserving diffusor layer.
The reflection polarizer is sometimes reflecting (specularly reflecting), which is annoying to the viewer. This reflecting effect is precluded by providing a diffusor which (as a result of its polarization-preserving character) preserves the above-described effect.
The same result can be achieved by means of a diffusor layer which is situated on the side of the first polarizer facing away from the liquid crystal. This diffusor layer does not have to preserve the polarization.
In a preferred embodiment, the diffusor layer serves as the birefringent layer.
Preferably, the directions of polarization of the reflective polarizer and the second polarizer cross each other substantially at right angles. This results in an optimum reflection of ambient light.
The reflective polarizer may further comprise at least one further retardation layer. Dependent upon the choice of the refractive indices of the retardation layers, the angular distribution of reflected light can be influenced or the polarization-changing effect increased, or other advantages can be achieved. A further display device for directed reflection (at a specific angle), which is predominantly used in portable (hand-held) applications, such as telephones, is characterized in that the optical axis of the reflective polarizer is at least locally tilted relative to the plane of the second polarizer.
A reflective polarizer according to the invention is characterized in that the reflective polarizer is provided at said first surface with a polarization-preserving diffusor layer and the reflective polarizer on a second surface facing away from the first surface has a polarization changing effect to light incident at an angle relative to the normal to the second surface.